1. Field of the Invention
This invention relates to tires for agricultural machines, construction and mining vehicles and more particularly to a lug tire provided with lugs projected from a tread surface and laid in herringbone pattern.
2. Description of the Prior Art
The tire for agricultural machines and the like serves to effect a traction operation on off road contrary to a tire for passenger cars or trucks and buses. As a result, the tire mounted on a driving wheel of the vehicles is provided with bar-shaped lugs projected from the tread center portion and extending toward both ends of the tread, the lugs being spaced apart from each other in the circumferential direction of the tire by transverse grooves. The height of the lug of the tire for agricultural machines and the like is considerably higher than that of the tire for trucks, buses or the like.
As well known in the art, in the case of manufacturing the tire for agricultural machines and the like, a carcass having a necessary construction is formed by laminating cord layers on a cylindrical former and to both end portions of the cord layers are secured bead wire rings. Subsequently, a tread rubber and side rubber each having a uniform thickness in the circumferential direction are superimposed one upon the other about the carcass to form a cylindrical green tire. The cylindrical green tire thus formed is put in a metal mold provided at its inner surface with indentations corresponding to outer shape of the tire and subjected to a vulcanization molding operation at a high temperature under pressure to provide a highly resilient tire.
In the case of the tire for the agricultural machines and the like belonging to the above described high lug tire, the recesses of the mold corresponding to the lugs are especially deep. As a result, the above mentioned tread crude rubber having a uniform thickness of the green tire is flowed into these recesses when the green tire is subjected to the vulcanization molding operation to form the lug portions and groove portions.
It is usual that the height of the lug of the tire is increased from the tread center portion to both end portions of the tire. It is a matter of coures, therefore, that the recess in the mold is correspondingly deep and wide at both the end portions of the tire.
The cylindrical green tire is urged against the inner wall of the mold and toroidally deformed by means of an inflated flexible bladder and vulcanized by steam introduced into the bladder. In the initial vulcanization and shaping step, the flexible bladder functions to apply its pressure in the first place to the tread center portion of the tire and subsequently to both the end portions of the tread owing to the tire shape and mold mechanism. As a result, the tread rubber in the green tire becomes heated and softened and a large amount of softened rubber flows from the tread center portion to the both end portions thereof. As a result, the skid base rubber corresponding to the groove base rubber becomes thicker than a required thickness at both the tread end portions and tends to be thinner than the required thickness at the tread center portion. It is, however, necessary to manufacture a tire which is not defective in ability and manufacturing cost.